Inflorescence Modes of pollination and Life cycle of Angiosperms (Lecture 7 & 8)
Lecture 7-8
Outline
- Types of inflorescence
- Mechanisms facilitating self & cross pollination
- Growth of the pollen tube and double fertilization
- Life cycle of angiosperm
- The evolutionary advantages of seeds
- Fruit development & Classification of fruits
- Evolutionary links between Angiosperm and animals
- Products from seed plants
3 Major Types of Inflorescence
1. Racemose Inflorescence (Indeterminate)
2. Cymose Inflorescence (Determinate)
3. Compound
- Special types of inflorescence
1.Hypanthodium.
2.Cyathium
3.Verticillaster
- Raceme: In such cases the main axis remains elongated and it bears laterally a number of stalked flowers e.g. Brassica
- Spike: In this type of racemose inflorescence the main axis remains elongated and the flowers are sessile e.g. amaranth
- Spikelet: Each spikelet may bear one to several flowers (florets) attached to a central stalk known as rachilla e.g. wheat
- Catkin: This is a modified spike with a long and drooping axis bearing unisexual flowers, e.g., mulberry, oak
- Spadix: This is also a modification of spike inflorescence having a fleshy axis, which remains enclosed by one or more large, often brightly coloured bracts, the spathes, e.g., in members of Araceae, Musaceae and Palmaceae.
- Corymb: In this inflorescence the main axis remains comparatively short and the lower flowers possess much longer stalks or pedicels than the upper ones so that all the flowers are brought more or less to the same level, e.g., in candytuft
- Umbel: In this inflorescence the primary axis remains comparatively short, and it bears at its tip a group of flowers which possess pedicels or stalks of more or less equal lengths so that the flowers are seen to spread out from a common point e.g., in coriander, fennel, carrot.
- Capitulum or Head: In this type of inflorescence the main axis or receptacle becomes suppressed, and almost flat, and the flowers (also known as florets) are sessile (without stalk) so that they become crowded together on the flat surface of the receptacle e.g. sunflower, marigold, safflower etc
Inflorescence: Type # 2. Cymose Inflorescence:
*In this type of inflorescence the growth of the main axis is ceased by the development of a flower at its apex,
and the lateral axis which develops the terminal flower 
also culminates in a flower and its growth is also ceased.
The flowers may be pedicellate (stalked) or sessile (without stalk).
*Here the flowers develop in basipetal succession,
i.e., the terminal flower is the oldest and the lateral ones younger.
e.g. Dianthus, Hamelia patents
Inflorescence: Type # 3. Compound Inflorescence:
OIn this type of inflorescence the main axis (peduncle) branches repeatedly once or twice in racemose or cymose manner. In the former case it becomes a compound raceme and in the latter case it becomes a compound cymose inflorescence.
*Compound Raceme or Panicle 
*Compound Cyme
*Compound Umbel
*Compound Spike
*Compound Corymb
Hypanthodium
*This is a special type of inflorescence.
*Here the receptacle is fleshy and 
forms hollow ball like structure with an apical opening.
*Three types of flowers develop on the inner surface of the receptacle.
*The female flowers are towards the base,
male flowers are towards the orifice
and short styled sterile female flowers are in between, e.g., Ficus sp.
*The inflorescence is pollinated by a gall wasp (Blastophaga).
Cyathium
*Flowers are of one sex, with male and female flowers usually borne on the same plant.
Petals are rarely present. 
Flowers of Euphorbia are in cup-shaped clusters called cyathia,
each of which seems to be a single female flower,
consisting of a single pistil surrounded by several male flowers,
each of which has a single stamen.
These clusters of reduced flowers are enclosed by an involucre.
Verticillaster
*A verticillaster is a mixed compound inflorescence 
with pairs of sessile or subsessile dichasial cymes
arranged in a false whorl of flowers (verticil).
*A verticil is a tightly packed group of cymes
that form a false whorl of flowers.
Coenanthium
It has an open saucer-shaped receptacle bearing florets as in hypanthodium, e.g., Dorstenia. 
Importance of Inflorescence
- It makes flowers more conspicuous to pollinating agents (insects/birds) so that chances of cross-pollination are high.
- A single pollinating agent can pollinate a number of flowers in a single visit.
- Inflorescence usually occurs away from vegetative parts and thus avoids hindrances for the pollinating agencies.
- A large number of pollens are shed from a group of flowers in the inflorescence so that air pollination becomes easier.
- As more flowers are pollinated at a time, more fruits are produced.
Mode of Pollination
- Pollination refers to the transfer of pollen grain from anthers to stigma.
- Pollen from an anther may fall on the stigma of the same flower leading to self pollination or autogamy.
- Sometimes pollen from an anther may fall on the stigma of another flower of different plants leading to cross pollination or allogamy/xenogamy.
- Some times pollen from an anther fall on the stigma of the another flower of the same plant leading to the geitonogamy.
1) Self Pollination:
It is the transfer of pollens to the stigma within the same flower, is always found in bisexual flower. In most of these species self pollination is not completed and cross pollination may occur up to 5%. There are various mechanisms that promote/facilitate self pollination.
Inbreeding = selfing
autogamy (w/in 1 flower) & geitonogamy (between fls. of 1 indiv.)
Selective advantage: ensures propagule production
Disadvantage: reduced to absent genetic variability
allautogamy: both outcrossing & inbreeding
e.g., Viola, Clarkia: two flower types:
chasmogamous flowers – normal, open
cleistogamous flowers – remain closed
Mechanisms facilitating self pollination
i) Bisexuality:
*Male and female sexual organs present in the same flower e.g. Wheat, rice, groundnut etc.
ii) Homogamy: 
Male and female sexual organs mature at the same time
e.g. Wheat, Groundnut etc..
iii) Cleistogamy:
*In this condition flowers does not open at all and ensure
complete self pollination. e.g. Oat, Barley, Grasses etc.
iv) Chasmogamy: 
In some species flower open but only after pollination
has taken place. A chasmogamous flower opens at maturity,
exposing stamens and style to allow fertilization.
e.g in many cereals and Benghal dayflower (Commelina benghalensis, tropical spiderwort)
v) In crop like Tomato and Brinjal stigma are closely surrounded by anthers, hence pollination occurs after opening of flower but the position of anther in relation to stigma ensure self pollination.
vi) In few species stigmas become receptive and elongate through staminal column, ensure self pollination.
vii) In crop like pea, bean, soybean, the flower open but stigma and anther are hidden by floral organs and ensures self pollination 
Preventing self-pollination
*Various mechanisms 
*stamens & carpels may mature at different times
*arranged so that animal pollinator won’t transfer
pollen from anthers to stigma of same flower
*biochemical self-incompatibility = block pollen tube growth
2) Cross pollination:
*The transfer of pollen from a flower to the stigma of the other flower of different plant. In cross pollination species pollination may brought about by wind, water, insect or animals.
Wind (anemophily), water (hydrophily), insect (entomophily), animal (Zoophily), Birds (ornithophily)
*In most of the cross pollinated species e.g. Bajra, maize, sunflower, alfalfa, castor, cross pollination is not complete and self pollination may occurs 5-10%. There are several mechanisms that facilitate cross pollination
Mechanisms facilitating cross pollination
1) dioecy in plants(incl. gynodioecy, androdioecy)
2) Difference in timing of floral parts = dichogamy
protandry – male flower first
protogyny – female flower first
3) Diferent lengths of style: Heterostyly
4) Spatial separation of anthers and stigmas = herkogamy
5) Self incompatability
What happens after landing pollen on a receptive stigma?
*Pollination is the transfer of pollen from an anther to a stigma
*A pollen grain germinates and produces a pollen tube that extends down between the cells of the style toward the ovary
The pollen tube
*Then discharges two sperm into the embryo sac
- Pollination enables gametes to come together within a flower
In angiosperms, the dominant sporophyte
- Produces spores that develop within flowers into male gametophytes (pollen grains)
- Produces female gametophytes (embryo sacs)
From Seed to Seedling
*Imbibition 
*Uptake of water
*Enzymes digest endosperm
- 1st organ = root
- 2nd organ = shoot
Structure & Function of the Seed
- Seed coat
*Protective layer around the seed
- Hypocotyl
*Below cotyledons
*Embryonic root
- Epicotyl
*Above cotyledons
*Embryonic shoot
The Evolutionary Advantage of Seeds
*In addition to seeds, the following are common to all seed plants
- Reduced gametophytes
- Heterospory
- Ovules
- Pollen
Fruits are classified into several types
Depending on their developmental origin
Coevolution of plants & animals
ONatural selection reinforced the interactions 
because they improved the reproductive success of both partners
Flowers and pollination
*A major advantage of flowers is that they have allowed angiosperms to use other organisms to move their pollen about.
*Bees, bats, birds and others all transport pollen. They are attracted to flowers by the nectar and pollen provided by the plant and when they visit multiple flowers they move pollen from one to the next
Evolutionary Links Between Angiosperms and Animals
*Pollination of flowers by animals and transport of seeds by animals
*Are two important relationships in terrestrial ecosystems
Types of Seed
Do seeds germinate as soon as they are dispersed
Based on the number of cotyledons present two types of seeds are recognized.
- i.Dicotyledonous seed: Seed with two cotyledons.
- Monocotyledonous seed: Seed with one cotyledon.
- Based on the presence or absence of the endosperm the seed is of two types.
- i. Albuminous or Endospermous seed:
The cotyledons are thin, membranous and mature seeds have endosperm persistent and
nourishes the seedling during its early development. Example: Castor, sunflower, maize.
- Ex-albuminous or non-endospermous seed:
Food is utilized by the developing embryo and so the mature seeds are without endosperm.
In such seeds, cotyledons store food and become thick and fleshy. Example: Pea, Groundnut.
Significance of Seeds:
The seed encloses and protects the embryo for next generation.
It contains food for the development of embryo.
It is a means for the dispersal of new individuals of the species.
A seed is a means for perpetuation of the species.
It may lie dormant during unfavorable conditions but germinates on getting suitable conditions.
Seeds of various plants are used as food, both for animals and men.
They are the basis of agriculture.
Seeds are the products of sexual reproduction so they provide genetic variations and recombination in a plant.
Products from Seed Plants
*Humans depend on seed plants for 
*Food
*Wood
*Many medicines
